The overall objective of this proposal is to elucidate the mechanism by which pituitary hormones acting on the testis regulate the development and maintenance of normal Leydig cell function. We propose to investigate the regulation of both testosterone production and estrogen production. Studies planned or in progress include: 1) the effect of in vivo treatment with gonadotropins on Leydig cell LH receptor concentration and on in vitro testosterone production in response to hCG in different populations of Leydig cells; 2) the role of Leydig cell estradiol synthesis in regulating testosterone production; 3) comparison of morphological and biochemical characteristics of the different populations of Leydig cells.